Depicted in FIG. 1 is an ink jet colour printer on which the main parts are labelled as follows: a fixed structure 41, a scanning carriage 42, an encoder 44 and printheads 40 which may be either monochromatic or colour, and variable in number.
The printer may be a stand-alone product, or be part of a photocopier, of a plotter, of a facsimile machine, of a machine for the reproduction of photographs and the like. The printing is effected on a physical medium 46, normally consisting of a sheet of paper, or a sheet of plastic, fabric or similar.
Also shown in FIG. 1 are the axes of reference:
x axis: horizontal, i.e. parallel to the scanning direction of the carriage 42; y axis: vertical, i.e. parallel to the direction of motion of the medium 46 during the line feed function; z axis: perpendicular to the x and y axes, i.e. substantially parallel to the direction of emission of the droplets of ink.
FIG. 2 shows an axonometric view of the printhead 40, on which are indicated nozzles 56, generally arranged in two columns parallel to the y axis, and a nozzle plate 106.
The composition and general mode of operation of a printhead according to the thermal type technology, and of the “top-shooter” type in particular, i.e. those that emit the ink droplets in a direction perpendicular to the actuating assembly, are already widely known in the sector art, and will not therefore be discussed in detail herein, this description instead dwelling more fully on only those features of the heads and the head manufacturing process of relevance for the purposes of understanding this invention.
FIG. 3 depicts a section parallel to the plane z-x of a head 40, which shows an ejector 55 corresponding to one of the nozzles 56. The following can be seen labelled: a tank 103 containing ink 142, a slot 102, a duct 53 of length G, a chamber 57, a resistor 27, a droplet 51 of ink, a bubble 65 of vapour, a meniscus 54 local to the surface of separation between ink and air, an external edge 66 and arrows 52 indicating the prevalent direction of motion of the ink.
FIG. 4 shows an enlarged axonometric view of two chambers 57, adjacent to and communicating with the slot 102 through the ducts 53, which generally are of rectangular section with depth h and width c.
The current technological trend in ink jet printheads is to produce a large number of nozzles per head (≧300), a definition of more than 600 dpi (dpi=dots per inch), a high working frequency (≧10 kHz) and smaller droplets (≦10 pl) than those produced in earlier technologies.
Requirements such as these are especially important in colour printhead manufacture and make it necessary to produce actuators and hydraulic circuits of increasingly smaller dimensions, greater levels of precision, and strict assembly tolerances.
These drawbacks are solved, for instance, by means of the monolithic printhead described in the Italian patent application TO 99A 000610, a section of which is illustrated in FIG. 5. A lamina 64, having width J and consisting of numerous layers, comprises the resistor 27 which, when a current passes through it, produces the heat needed to form the vapour bubble 65 which, by expanding rapidly inside the chamber 57, results in emission of the droplet of ink 51 through the nozzle 56. The lamina 64 is of a thickness generally between 1 and 50 mμ, and is subject to vibration on account of the sudden formation and subsequent collapse of the vapour bubble 65.
In the patents U.S. Pat. No. 6,000,787 and EP 0 936 070, heads are described the chambers of which are produced laterally with respect to the grooves: as a result, the resistors are adherent to a body of Silicon having a much greater thickness than that of the lamina and therefore exempt from the above-mentioned vibrations. However the solutions described in the patents quoted do not solve the problem described below.
It is in fact important to ensure that the volume and speed of the droplets successively emitted are as constant as possible, and that no “satellite” droplets are formed as these, with a trajectory generally different from the main droplets, are distributed randomly near the edges of the graphic symbols, reducing their sharpness.
This problem is solved, for instance, by means of the head with multiple ink feeder channels described in the Italian patent application AO 99A 0002. FIG. 6 illustrates an ejector 55′ of this printhead, comprising the slot 102, the chamber 57, the resistor 27 and elementary ducts 72, which convey the ink 142 from the slot 102 to the chamber 57, each of which having depth a, width b and length g. By way of example, the figure shows three elementary ducts 72, but their number N could be different from this.
The patent quoted above discloses the details of a method for calculating the width b and the number N which permit to render minimal the time constant τ of the column of ink that fills the ejector 55′ and at the same time to render critical the damping of the oscillations of the meniscus 54 following emission of the droplet 51, for the purposes of obtaining a high emission frequency of the droplets, of ensuring that their volume and speed are as constant as possible, and of avoiding the formation of satellite droplets. This head, however, is not monolithic.
A further problem found in thermal ink jet printheads will now be illustrated. If various ejectors are driven simultaneously, to print for instance a vertical line, some of the tracks belonging to the microelectronics are passed through by the sum of the driving currents. This sum varies in function of the number of ejectors activated on each occasion, and in turn produces a variable voltage drop.
It is preferable to command the ejectors at different times, so that only the current needed for a single ejector passes through the tracks each time and the voltage drops on each are therefore small and constant. In addition, it is necessary for any two ejectors that are immediately successive in the time sequence of commands not to be adjacent, the purpose being to avoid the phenomenon of intermodulation, known to those acquainted with the sector art.
For these reasons, the ejectors belonging to each column parallel to the y axis are staggered progressively by an interval parallel to the x axis. Compensation for the mechanical stagger is provided by a corresponding time delay in the commands, with the purpose of obtaining the desired figure from printing.
On account of the mechanical stagger, the length G of the duct 53, or the length g of the elementary ducts 72, are different for the different ejectors, with a consequent variation in the titne constant τ and in criticality, among the ejectors, of the damping of the oscillations of the meniscus 54.